Using monoclonal anti-Tac antibody, the human receptor for T-cell growth factor (TCGF, interleukin-2) has been characterized and purified to homogeneity. This receptor is a 55,000 dalton glycoprotein with internal disulfide bonds and an isoelectric point of 5.4 - 5.7. The receptor is composed of 33,000 daltons of peptide backbone and is post-translationally modified by introduction of N-and O-linked carbohydrate as well as sialic acid yielding mature receptor. The receptor does not appear to be phosphorylated, sulfated, nor covalently linked to palmitic acid. TCGF receptor expression has been evaluated in varying lymphoid malignancies. The adult T cell leukemia which is etiologically associated with type C retrovirus infection (human T cell leukemia/lymphoma virus, HTLV) consistently displays large numbers of TCGF receptors which may be aberrant in size. In contract, Sezary leukemic T cells and acute lymphocytic T leukemic cells do not contain these receptors. Select acute lymphocytic leukemias however can be induced to differentiate and exhibit TCGF receptors using phorbol esters. The monoclonal antibody is now being used to aid in the diagnosis of these leukemias as well as being evaluated for potential therapeutic effects in patients with adult T cell leukemia. Hairy cell leukemia, and select Burkitt lymphomas also express TCGF receptors as defined by anit-Tac. These leukemias are of B cell origin, yet unexpectedly exhibit small numbers of receptors recognized by anti-Tac. Monoclonal anti-Tac has also been used to modulate the normal human immune response. Anti-Tac markedly inhibits 1) in vitro T cell proliferation induced by soluble, allogeneic and autologous antigen, 2) the maturation of cytotoxic T cells, and 3) T cell dependent B cell immunoglobulin production.